Changes in the epidermal basal cell density (BCD) in human skin were determined during and immediately after fractionate radiotherapy. The basal cells are one of the target cell types responsible for acute skin reactions and measuring the BCD is a histological method for studying the time course and degree of reaction. Thirty-two patients with breast cancer participated in this study. They received postmastectomy radiotherapy to the thoracic wall. A 3-mm punch biopsy was taken from the irradiation field once a week for 6-10 weeks and the linear basal cell density was determined. Standard fractionation at two different dose levels (40 and 50 Gy) as well as hypofractionation and accelerated treatment have been investigated. For the first 3 weeks we found a constant decline in the BCD (about 0.8%/day), independent of dose and fractionation schedule. Using the nadir value as endpoint we found a dose-response relationship between 40 and 50 Gy, and for total effect (TE)-values in the range 430-1015. Compared to standard fractionation, hypofractionation showed somewhat less effect and accelerated fractionation showed significantly less effect. The reduced effect of accelerated fractionation is interpreted as a result of lack of cell cycle redistribution of cells between the two daily fractions in this type of tissue. The removal rate of dead or doomed cells could also affect the results for schedules with different overall time. The results of BCD were also compared to erythema.(ABSTRACT TRUNCATED AT 250 WORDS)